The invention relates to a drive device for a motor vehicle, comprising a differential for distributing a drive torque that can be supplied via a drive shaft to two output shafts, and a superposition gear coupled to the differential, one of the output shafts and an additional motor, in order to superimpose the torque supplied by the output shaft, wherein the differential is coupled via a torque reducing transmission device to the superposition gear.
Differentials are used in motor vehicles in order to distribute a driving torque provided by a drive motor to different wheels of an axle, or to different axles. In order to prevent twisting of wheels and to transmit yaw moments to the motor vehicle in a specific driving situation, it is advantageous to influence in a certain way the torque distribution on the output shafts of such a differential. One possibility for this is so called electronic torque vectoring, in which an additional motor is used in order to distribute the torques to the output shafts of the differential. Torque vectoring in this case describes in this case the distribution of the torque both for faster rotation and also for slower rotation of the wheel of a vehicle axle.
From publication WO 2006/010186 A1 is known an electric torque vectoring transmission in which a driving torque is distributed via two bevel differentials to two drive shafts. Superposition of an additional electromotor is carried out with two planetary gears, to achieve that the electromotor is stationary when the wheels are rotating at the same speed. The problem in this case is that due to the use of a bevel gear differential, a high transmission ratio of the electromotor as well as many transmission gear ratios are required and that the load on the planetary gears is relatively high with the described arrangement.
Another possibility for designing an electrical torque vectoring transmission is described in the publication DE 2005 049 707 A2. Parts of the superposition gear are in this case integrated in the differential of the motor vehicle. A disadvantage of the described arrangement is that the planetary gears constantly roll. In order to achieve a standstill of the electromotor at equally fast high rotational speeds of the drive shafts, an additional parallel shaft must be provided.
The publication U.S. Pat. No. 5,387,161 also discloses an electrical torque vectoring transmission. The differential is in this case formed by a spur wheel planetary gear. The transmission gear is coupled to the differential via a spur wheel stage and comprises a planetary gear. In particular, the spur wheel stage is with the described arrangement exposed to high loads, which is why a corresponding dimensioning of the components is required.
In order to reduce high loads of a superposition gear with electronic torque vectoring, publication WO 2008/125364 A proposes an additional planetary gear for torque reduction between a superposition gear and the differential. This can be designed in particular in the form of a Ravigneaux gear set together with the differential of the motor vehicle. In this case, the load on the superposition gear is approximately one fourth of the wheel differential torque, which means that the transmission is small in size and that it can be easily constructed.
It is also known that an electromotor that is used within the context of electronic torque vectoring can be also used to provide hybrid functions in a motor vehicle. For example the application DE 10 2010 036 240 proposes to couple the electromotor selectively to the differential in such a way that a drive torque provided by the electromotor is distributed to the drive shafts of the differential and thus to the wheels of the motor vehicle, or to couple the electromotor to the superposition gear in such a way that is usable for torque vectoring. The complete decoupling of the torque path for a hybrid operation on the one hand, and for a torque vectoring on the other hand, leads to a relatively complicated construction type, wherein additional planetary or spur wheel transmissions are generally required in order to achieve the desired transmission ratios in the hybrid or torque vectoring mode. In addition, the load on the components of the superposition gear is relatively heavy.
The application DE 10 2006 031 089 discloses a torque vectoring transmission wherein a gear train for the hybrid mode is constructed at least partially jointly for a torque vectoring mode. The switching between different operating modes is carried out by shifting of a shift sleeve. The design of the transmission is relatively complicated, in particular because stepped planetary gears are required. In addition, the load on the superposition gear is relatively high in the torque vectoring mode. Moreover, the transmission ratios between the electromotor and the wheel differential mode in the torque vectoring mode or in the driving torque in the hybrid mode are the same. However, different transmission ratios are typically used in different modes.